Conditional compile generic lambda (#381)

ThomsonTan · web-flow · commit ca5eed056eb7 · 2020-10-30T11:00:16.000-07:00
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -30,7 +30,7 @@ endif()
 
 find_package(Threads)
 
-if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+if(MSVC)
   # Options for Visual C++ compiler: /Zc:__cplusplus - report an updated value
   # for recent C++ language standards. Without this option MSVC returns the
   # value of __cplusplus="199711L"
diff --git a/exporters/ostream/include/opentelemetry/exporters/ostream/span_exporter.h b/exporters/ostream/include/opentelemetry/exporters/ostream/span_exporter.h
@@ -89,6 +89,9 @@ class OStreamSpanExporter final : public sdktrace::SpanExporter
     sout_ << ']';
   }
 
+// Prior to C++14, generic lambda is not available so fallback to functor.
+#if __cplusplus < 201402L
+
   class SpanDataAttributeValueVisitor
   {
   public:
@@ -104,9 +107,15 @@ class OStreamSpanExporter final : public sdktrace::SpanExporter
     OStreamSpanExporter &exporter_;
   };
 
+#endif
+
   void print_value(sdktrace::SpanDataAttributeValue &value)
   {
+#if __cplusplus < 201402L
     nostd::visit(SpanDataAttributeValueVisitor(*this), value);
+#else
+    nostd::visit([this](auto &&arg) { print_value(arg); }, value);
+#endif
   }
 
   void printAttributes(std::unordered_map<std::string, sdktrace::SpanDataAttributeValue> map)

Original file line number	Diff line number	Diff line change
`@@ -89,6 +89,9 @@ class OStreamSpanExporter final : public sdktrace::SpanExporter`
`89`	`89`	`sout_ << ']';`
`90`	`90`	`}`
`91`	`91`
	`92`	`+// Prior to C++14, generic lambda is not available so fallback to functor.`
	`93`	`+#if __cplusplus < 201402L`
	`94`	`+`
`92`	`95`	`class SpanDataAttributeValueVisitor`
`93`	`96`	`{`
`94`	`97`	`public:`
`@@ -104,9 +107,15 @@ class OStreamSpanExporter final : public sdktrace::SpanExporter`
`104`	`107`	`OStreamSpanExporter &exporter_;`
`105`	`108`	`};`
`106`	`109`
	`110`	`+#endif`
	`111`	`+`
`107`	`112`	`void print_value(sdktrace::SpanDataAttributeValue &value)`
`108`	`113`	`{`
	`114`	`+#if __cplusplus < 201402L`
`109`	`115`	`nostd::visit(SpanDataAttributeValueVisitor(*this), value);`
	`116`	`+#else`
	`117`	`+ nostd::visit([this](auto &&arg) { print_value(arg); }, value);`
	`118`	`+#endif`
`110`	`119`	`}`
`111`	`120`
`112`	`121`	`void printAttributes(std::unordered_map<std::string, sdktrace::SpanDataAttributeValue> map)`